Studies on the replication of Rts 1 DNA: We shall extend our present observation that the thermosensitive lesion of the drug resistance factor Rts 1 is at the formation of closed covalent circle. We shall attempt to obtain conversion of the linear form of Rts 1 to the circular form in vitro and characterize the enzymes which catalyze this process. Studies on the physiological effect of Rts 1 at the non-permissive temperature: We shall attempt to elucidate the molecular basis of our observation that Rts 1 at the non-permissive temperature does not allow an increase in viable cell number to increase even though total number increases linearly. From our preliminary studies and studies by other laboratories, three important facts are known. They are: a) At non-permissive temperature some change in host membrane takes place; b) The existence of Rts 1 give restriction to T4 phage multiplication, while at non-permissive temperature this restriction breaks down; and c) at non-permissive temperature, formation of closed covalent circles of Rts 1 plasmid DNA is inhibited while the replication of the plasmid DNA takes place. We shall attempt to elucidate the relationships between these facts and the effect of Rts 1 on the growth of host cells as described above. BIBLIOGRAPHIC REFERENCES: Dijoseph, C.G. and Kaji, A. Change in the cell envelope of E. coli carrying the thermosensitive drug resistance factor, Rts 1 at the non-permissive temperature. Anti-microbial Agents and Chemotherapy 8:504 (1975). Yamamoto, T., T. Yokota, and A. Kaji. The role of cyclic AMP in the thermosensitive lesion of the formation of closed covalent circular Rts 1 DNA. Biochem. Biophys. Res. Commun. 62:548 (1975).